Desiccant wheels are used primarily for drying air in industrial or commercial applications specifically where low humidity is required. Since dry air is the key requirement, large wheels are combined with substantial high temperature regeneration heating to obtain very dry, warm air which may or may not be cooled by other equipment downstream. However, for application to building air-conditioning, both drying and cooling of air are important, and energy efficiency is paramount. Where a process air stream is dried in a desiccant wheel it also undergoes heating due to both the exothermic adsorption process and the carry-over of heat from the regeneration side of the wheel via the wheel thermal mass. This heating limits both the amount of dehumidification that can be achieved, and also makes the exiting process air hotter thus limiting the minimum temperature which can be achieved even after subsequent evaporative cooling.
This has led researchers to propose multi-wheel intercooled desiccant cycles [Desiccant properties and their affect on cooling system performance. Collier, R. 1989, ASHRAE Transactions, Vol. 1, pp. 823-827.], multi-stage intercooled cycles [Technical development of rotary desiccant dehumidification and air conditioning: a review La, D., et al. 2010, Renewable and Sustainable Energy Reviews, Vol. 14, pp. 130-147.] and a wheel allowing integrated cooling [Double-stage dehumidification in a two-rotor desiccant cooling process equipped with a multi-divided adsorbent rotor. Kodama, A., et al. 12, Japan: Japan Society of Chemical Engineers, 2009, Journal of Chemical Engineering of Japan, Vol. 42, pp. 930-936.]. However, in the case of solid desiccant wheels, incorporating process air cooling in the rotating desiccant matrix naturally leads to a more complicated design than when a batch process is used and demonstration of improved performance is yet to be achieved. In “Performance of a multipass honeycomb adsorber regenerated by a direct hot water heating. Kodama, A., et al. s.l.: Springer, 2005, Adsorption, Vol. 11, pp. 603-608” the authors have developed a multipass cross-flow desiccant wheel design incorporating process air cooling via a cooling air flow in separate channels as well as regeneration side heating with additional hot water channels. In their design, cooling air flows along the axial direction and the process and regeneration air streams flow into and out of the wheel in a direction perpendicular to the wheel axis. However, the authors found that the performance of the wheel was less than expected due to the high heat capacity of the wheel structure.
An internally cooled wheel design based on a parallel plate type arrangement has been proposed by Narayanan, R., Saman, W., & White, S. (2013). A non-adiabatic desiccant wheel: modelling and experimental validation. Applied Thermal Engineering, (61), 178-185.). In their design, cooling air enters in the axial direction through the wheel hub and exits the wheel in alternate channels perpendicular to the axis. Although this wheel was not constructed, testing results in a single channel suggest a significant increase in dehumidification performance when cooling was activated.
Despite the increase in dehumidification performance there is still scope for further improvements to desiccant wheel design and operation.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.